In my experience it isn't really about quad overdraw/helper lanes, but rather about primitive assembly. On my RTX 4090, vertex shaders max out at about 20M tris/ms, while mesh shaders, which bypass primitive assembly, can reach 40M tris/ms. Maybe quad draw might be the next bottleneck after primitive assembly.

But it's a bit difficult to find the exact reason since profilers won't really tell you a concrete bottleneck when drawing vast amounts of tiny tris.